Processes for Producing Acrylic Acids and Acrylates

ABSTRACT

In one embodiment, the invention is to a process for producing an acrylate product. The process comprises the step of providing a crude product stream comprising the acrylate product, an alkylenating agent, and water. The process further comprises the step of contacting at least a portion of the crude product stream or a derivative thereof with an extraction agent mixture to form an extract stream and a raffinate stream. The extract stream comprises acrylate product and extraction agent. The raffinate stream comprises alkylenating agent and water. Preferably, the extraction agent mixture comprises at least two extraction agents.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. application Ser. No.13/251,623, filed on Oct. 3, 2011 and to U.S. application Ser. No.13/327,888, filed on Dec. 16, 2011, the entireties of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to the production of acrylicproducts, including acrylic acid and/or acrylates. More specifically,the present invention relates to the separation of acrylic acid fromformaldehyde formed via the condensation of acetic acid andformaldehyde.

BACKGROUND OF THE INVENTION

α,β-unsaturated acids, particularly acrylic acid and methacrylic acid,and the ester derivatives thereof are useful organic compounds in thechemical industry. These acids and esters are known to readilypolymerize or co-polymerize to form homopolymers or copolymers. Oftenthe polymerized acids are useful in applications such assuperabsorbents, dispersants, flocculants, and thickeners. Thepolymerized ester derivatives are used in coatings (including latexpaints), textiles, adhesives, plastics, fibers, and synthetic resins.

Because acrylic acid and its esters have long been valued commercially,many methods of production have been developed. One exemplary acrylicacid ester production process utilizes: (1) the reaction of acetylenewith water and carbon monoxide; and/or (2) the reaction of an alcoholand carbon monoxide, in the presence of an acid, e.g., hydrochloricacid, and nickel tetracarbonyl, to yield a crude product comprising theacrylate ester as well as hydrogen and nickel chloride. Anotherconventional process involves the reaction of ketene (often obtained bythe pyrolysis of acetone or acetic acid) with formaldehyde, which yieldsa crude product comprising acrylic acid and either water (when aceticacid is used as a pyrolysis reactant) or methane (when acetone is usedas a pyrolysis reactant). These processes have become obsolete foreconomic, environmental, or other reasons.

More recent acrylic acid production processes have relied on the gasphase oxidation of propylene, via acrolein, to form acrylic acid. Thereaction can be carried out in single- or two-step processes, but thelatter is favored because of higher yields. The oxidation of propyleneproduces acrolein, acrylic acid, acetaldehyde and carbon oxides. Acrylicacid from the primary oxidation can be recovered while the acrolein isfed to a second step to yield the crude acrylic acid product, whichcomprises acrylic acid, water, small amounts of acetic acid, as well asimpurities such as furfural, acrolein, and propionic acid. Purificationof the crude product may be carried out by azeotropic distillation.Although this process may show some improvement over earlier processes,this process suffers from production and/or separation inefficiencies.In addition, this oxidation reaction is highly exothermic and, as such,creates an explosion risk. As a result, more expensive reactor designand metallurgy are required. Also, the cost of propylene is oftenprohibitive.

The aldol condensation reaction of formaldehyde and acetic acid and/orcarboxylic acid esters has been disclosed in literature. This reactionforms acrylic acid and is often conducted over a catalyst. For example,condensation catalysts consisting of mixed oxides of vanadium andphosphorus were investigated and described in M. Ai, J. Catal., 107, 201(1987); M. Ai, J. Catal., 124, 293 (1990); M. Ai, Appl. Catal., 36, 221(1988); and M. Ai, Shokubai, 29, 522 (1987). The acetic acid conversionsin these reactions, however, may leave room for improvement. Althoughthis reaction is disclosed, there has been little if any disclosurerelating to separation schemes that may be employed to effectivelyprovide purified acrylic acid from the aldol condensation crude product.

Thus, the need exists for processes for producing purified acrylic acidand, in particular, for separation schemes to effectively purify uniquealdol condensation crude products to form the purified acrylic acid.

The references mentioned above are hereby incorporated by reference.

BRIEF DESCRIPTION OF DRAWINGS

The invention is described in detail below with reference to theappended drawings, wherein like numerals designate similar parts.

FIG. 1 is a process flowsheet showing an acrylic acidreaction/separation system in accordance with an embodiment of thepresent invention.

FIG. 2 is a schematic diagram of an alkylenating split of an acrylicacid reaction/separation system in accordance with one embodiment of thepresent invention.

FIG. 3 is a schematic diagram of an acrylic acid reaction/separationsystem in accordance with one embodiment of the present invention.

SUMMARY OF THE INVENTION

In one embodiment, the invention is to a process for producing anacrylate product. Preferably, the inventive process yields acrylic acid.The process comprises the step of providing a crude product streamcomprising acrylic acid and/or other acrylate products, an alkylenatingagent, and optionally water. The crude product stream may comprise atleast 1 wt % alkylenating agent. The alkylenating agent may be, forexample, formaldehyde. In preferred embodiments, the crude productstream is formed by contacting acetic acid and formaldehyde over acatalyst and under conditions effective to form the crude productstream. In one embodiment, the inventive process further comprises thestep of contacting at least a portion of the crude product stream or aderivative thereof with an extraction agent mixture comprising at leasttwo extraction agents to form an extract stream and a raffinate stream.The extract stream comprises acrylate product and extraction agent andthe raffinate stream comprises alkylenating agent, water, and optionallyextraction agent.

DETAILED DESCRIPTION OF THE INVENTION Introduction

Production of unsaturated carboxylic acids such as acrylic acid andmethacrylic acid and the ester derivatives thereof via most conventionalprocesses have been limited by economic and environmental constraints.In the interest of finding a new reaction path, the aldol condensationreaction of acetic acid and an alkylenating agent, e.g., formaldehyde,has been investigated. This reaction may yield a unique crude productthat comprises, inter alia, a higher amount of (residual) formaldehyde,which is generally known to add unpredictability and problems toseparation schemes. Although the aldol condensation reaction of aceticacid and formaldehyde is known, there has been little if any disclosurerelating to separation schemes that may be employed to effectivelypurify the unique crude product that is produced. Other conventionalreactions, e.g., propylene oxidation or ketene/formaldehyde, do notyield crude products that comprise higher amounts of formaldehyde. Theprimary reactions and the side reactions in propylene oxidation do notcreate formaldehyde. In the reaction of ketene and formaldehyde, atwo-step reaction is employed and the formaldehyde is confined to thefirst stage. Also, the ketene is highly reactive and convertssubstantially all of the reactant formaldehyde. As a result of thesefeatures, very little, if any, formaldehyde remains in the crude productexiting the reaction zone. Because no formaldehyde is present in crudeproducts formed by these conventional reactions, the separation schemesassociated therewith have not addressed the problems andunpredictability that accompany crude products that have higherformaldehyde content.

In one embodiment, the present invention is to a process for producingacrylic acid, methacrylic acid, and/or the salts and esters thereof. Asused herein, acrylic acid, methacrylic acid, and/or the salts and estersthereof, collectively or individually, may be referred to as “acrylateproducts.” The use of the terms acrylic acid, methacrylic acid, or thesalts and esters thereof, individually, does not exclude the otheracrylate products, and the use of the term acrylate product does notrequire the presence of acrylic acid, methacrylic acid, and the saltsand esters thereof.

The inventive process, in one embodiment, includes the step of providinga crude product stream comprising the acrylic acid and/or other acrylateproducts. The crude product stream of the present invention, unlike mostconventional acrylic acid-containing crude products, further comprises asignificant portion of at least one alkylenating agent. Preferably, theat least one alkylenating agent is formaldehyde. For example, the crudeproduct stream may comprise at least 0.5 wt. % alkylenating agent(s),e.g., at least 1 wt. %, at least 5 wt. %, at least 7 wt. %, at least 10wt. %, or at least 25 wt. %. In terms of ranges, the crude productstream may comprise from 0.5 wt. % to 50 wt. % alkylenating agent(s),e.g., from 1 wt. % to 45 wt. %, from 1 wt. % to 25 wt. %, from 1 wt. %to 10 wt. %, or from 5 wt. % to 10 wt. %. In terms of upper limits, thecrude product stream may comprise less than 50 wt. % alkylenatingagent(s), e.g., less than 45 wt. %, less than 25 wt. %, or less than 10wt. %.

In one embodiment, the crude product stream of the present inventionfurther comprises water. For example, the crude product stream maycomprise less than 60 wt. % water, e.g., less than 50 wt. %, less than40 wt. %, or less than 30 wt. %. In terms of ranges, the crude productstream may comprise from 1 wt. % to 60 wt. % water, e.g., from 5 wt. %to 50 wt. %, from 10 wt. % to 40 wt. %, or from 15 wt. % to 40 wt. %. Interms of upper limits, the crude product stream may comprise at least 1wt. % water, e.g., at least 5 wt. %, at least 10 wt. %, or at least 15wt. %.

In one embodiment, the crude product stream of the present inventioncomprises very little, if any, of the impurities found in mostconventional acrylic acid crude product streams. For example, the crudeproduct stream of the present invention may comprise less than 1000 wppmof such impurities (either as individual components or collectively),e.g., less than 500 wppm, less than 100 wppm, less than 50 wppm, or lessthan 10 wppm. Exemplary impurities include acetylene, ketene,beta-propiolactone, higher alcohols, e.g., C₂₊, C₃₊, or C₄₊, andcombinations thereof. Importantly, the crude product stream of thepresent invention comprises very little, if any, furfural and/oracrolein. In one embodiment, the crude product stream comprisessubstantially no furfural and/or acrolein, e.g., no furfural and/oracrolein. In one embodiment, the crude product stream comprises lessthan less than 500 wppm acrolein, e.g., less than 100 wppm, less than 50wppm, or less than 10 wppm. In one embodiment, the crude product streamcomprises less than less than 500 wppm furfural, e.g., less than 100wppm, less than 50 wppm, or less than 10 wppm. Furfural and acrolein areknown to act as detrimental chain terminators in acrylic acidpolymerization reactions. Also, furfural and/or acrolein are known tohave adverse effects on the color of purified product and/or tosubsequent polymerized products.

In addition to the acrylic acid and the alkylenating agent, the crudeproduct stream may further comprise acetic acid, water, propionic acid,and light ends such as oxygen, nitrogen, carbon monoxide, carbondioxide, methanol, methyl acetate, methyl acrylate, acetaldehyde,hydrogen, and acetone. Exemplary compositional data for the crudeproduct stream are shown in Table 1. Components other than those listedin Table 1 may also be present in the crude product stream.

TABLE 1 CRUDE ACRYLATE PRODUCT STREAM COMPOSITIONS Conc. Conc. Conc.Conc. Component (wt. %) (wt. %) (wt. %) (wt. %) Acrylic Acid   1 to 75  1 to 50   5 to 50   10 to 40 Alkylenating Agent(s)  0.5 to 50   1 to45   1 to 25   1 to 10 Acetic Acid   1 to 90   1 to 70   5 to 50   10 to50 Water   1 to 60   5 to 50   10 to 40   15 to 40 Propionic Acid 0.01to 10 0.1 to 10 0.1 to 5 0.1 to 1 Oxygen 0.01 to 10 0.1 to 10 0.1 to 50.1 to 1 Nitrogen  0.1 to 20 0.1 to 10 0.5 to 5 0.5 to 4 Carbon Monoxide0.01 to 10 0.1 to 10 0.1 to 5 0.5 to 3 Carbon Dioxide 0.01 to 10 0.1 to10 0.1 to 5 0.5 to 3 Other Light Ends 0.01 to 10 0.1 to 10 0.1 to 5 0.5to 3

The unique crude product stream of the present invention may beseparated in a separation zone to form a final product, e.g., a finalacrylic acid product. In one embodiment, the inventive process comprisesthe step of separating at least a portion of the crude product stream toform an alkylenating agent stream and an intermediate product stream. Ina preferred embodiment, the process comprises the step of contacting atleast a portion of the crude product stream or a derivative thereof withan extraction agent mixture comprising at least two extraction agents toform an extract stream comprising acrylate product and extraction agentand a raffinate stream comprising alkylenating agent and water. Aderivative of the crude product stream may be, in some embodiments, astream that is derived from the crude product stream, e.g., via aseparation process. Generally, this separating step may be referred toas an “akylenating agent split.” In one embodiment, the alkylenatingagent stream comprises significant amounts of alkylenating agent(s). Forexample, the alkylenating agent stream may comprise at least 1 wt. %alkylenating agent(s), e.g., at least 5 wt. %, at least 10 wt. %, atleast 15 wt. %, or at least 25 wt. %. In terms of ranges, thealkylenating stream may comprise from 1 wt. % to 75 wt. % alkylenatingagent(s), e.g., from 3 to 50 wt. %, from 3 wt. % to 25 wt. %, or from 10wt. % to 20 wt. %. In terms of upper limits, the alkylenating stream maycomprise less than 75 wt. % alkylenating agent(s), e.g. less than 50 wt.% or less than 40 wt. %. In preferred embodiments, the alkylenatingagent is formaldehyde.

As noted above, the presence of alkylenating agent in the crude productstream adds unpredictability and problems to separation schemes. Withoutbeing bound by theory, it is believed that formaldehyde reacts in manyside reactions with water to form by-products. The following sidereactions are exemplary.

CH₂O+H₂O→HOCH₂OH

HO(CH₂O)_(i-1)H+HOCH₂OH→HO(CH₂O)_(i)H+H₂O for i>1

Without being bound by theory, it is believed that, in some embodiments,as a result of these reactions, the alkylenating agent, e.g.,formaldehyde, acts as a “light” component at higher temperatures and asa “heavy” component at lower temperatures. The reaction(s) areexothermic. Accordingly, the equilibrium constant increases astemperature decreases and decreases as temperature increases. At lowertemperatures, the larger equilibrium constant favors methylene glycoland oligomer production and formaldehyde becomes limited, and, as such,behaves as a heavy component. At higher temperatures, the smallerequilibrium constant favors formaldehyde production and methylene glycolbecomes limited. As such, formaldehyde behaves as a light component. Inview of these difficulties, as well as others, the separation of streamsthat comprise water and formaldehyde cannot be expected to behave as atypical two-component system. These features contribute to theunpredictability and difficulty of the separation of the unique crudeproduct stream of the present invention.

The present invention, surprisingly and unexpectedly, achieves effectiveseparation of alkylenating agent(s) from the inventive crude productstream to yield a purified product comprising acrylate product and verylow amounts of other impurities.

As noted above, the alkylenating split, in preferred embodiments, iscarried out in an extraction unit, e.g., a liquid-liquid extractionunit, using an extraction agent mixture comprising at least twoextraction agents, e.g., at least three or at least four. Preferably,the inventive process comprises the step of contacting the crude productstream with the extraction agent mixture. The contacting step forms atleast one extract stream and at least one raffinate stream. The extractstream(s) comprise, inter alia, acrylate product, extraction agent, andacetic acid. The raffinate stream(s) comprise, inter alia, alkylenatingagent, extraction agent, water, and acetic acid. It has now beendiscovered that the use of multiple extraction agents surprisingly andunexpectedly provides for a more effective separation, as compared to anextraction unit employing a single extraction agent. As a result, theamount of alkylenating agent, acetic acid, and water extracted from thecrude product stream (and into the extract stream) is reduced. Theresultant extract stream, unexpectedly, comprises lower amounts ofalkylenating agent, acetic acid, and water, as compared to an extractstream formed via an extraction unit employing a single extractionagent.

Without being bound by theory, it is believed that the use of theinventive extraction agent mixture allows the components thereof tobetter correspond to the various components of the crude product stream.For example, one extraction agent in the extraction agent mixture maymore effectively attract acrylic acid while a different extraction agentin the extraction agent mixture may more strongly repel formaldehyde andwater. Thus, by selecting and employing multiple extraction agents eachhaving different propensities to attract or repel the particularcomponents of the crude product stream, a more effective separation isachieved.

In one embodiment, as a result of the inventive process, the extractstream(s) comprise alkylenating agent in an amount less than 54%, e.g.,less than 50% or less than 40%, of the total alkylenating agent that wasinitially present in the crude product stream. The inventive extractionagent mixture has a lesser selectivity to impurities, e.g., alkylenatingagent, acetic acid, water. As such, less of the impurities are extractedinto the extract stream. Conventional extraction techniques that employa single extraction agent allow higher percentages of impurities to beextracted into the extract stream. In one embodiment, as a result of theinventive process, the extract stream(s) comprise water in an amountless than 60%, e.g., less than 50% or less than 40%, of the total waterthat was initially present in the crude product stream. In oneembodiment, the extraction selectivity to alkylenating agent is lessthan 0.9, e.g., less than 0.5 or less than 0.1. Extraction selectivity,as used herein, refers to the ratio of the amount of a particularcomponent that is extracted to the amount of the same component that isnot extracted. For example, extraction selectivity may be the ratio ofthe weight percentage of a component in the extract stream to the weightpercentage of the component in the raffinate stream.

In one embodiment, the extract stream(s) comprise acrylate product in anamount greater than 75%, e.g., greater than 95% or greater than 97%, ofthe total acrylate product that was initially present in the crudeproduct stream.

In some embodiments, when the inventive extraction agent mixture isemployed, the extract stream(s) comprise less than 20 wt. % alkylenatingagent, e.g., less than 10 wt. %, less than 5 wt. %, less than 3 wt. %,or less than 1 wt. %. In terms of ranges, the extract stream(s)comprises from 0.01 wt. % to 20 wt. % alkylenating agent, e.g., from 0.1wt. % to 5 wt. % or from 0.1 wt. % to 1 wt. %.

In one embodiment, the extract stream(s) comprises less than 30 wt %water, e.g., less than 15 wt % water, less than 10 wt %, less than 5 wt%, or less than 3 wt %. In terms of ranges, the extract stream(s)comprises from 0.01 wt. % to 30 wt. % water, e.g., from 0.1 wt. % to 10wt. % or from 0.1 wt. % to 5 wt. %.

In one embodiment, the extract stream(s) comprise from 1 wt. % to 85 wt.% acetic acid, e.g., from 20 wt. % to 65 wt. % or from 30 wt. % to 55wt. %.

In some embodiments, weight percentages are calculated based on thetotal weight of the respective stream, excluding the weight of theextraction agents.

In some embodiments, the mass ratio of acrylate product to alkylenatingagent in the extract stream is greater than 5:1, e.g., greater than 7:1or greater than 10:1. In some embodiments, the mass ratio of acrylateproduct to alkylenating agent in the raffinate stream is less than0.05:1, e.g., less than 0.033:1 or less than 0.02:1.

In one embodiment, the raffinate stream(s) comprise less than 15 wt. %acrylate product, i.e., less than 10 wt. %, less than 5 wt. %, or lessthan 1 wt. %. In terms of ranges, the raffinate stream(s) comprise from0.01 wt. % to 15 wt. % acrylate product, e.g., from 0.1 wt. % to 10 wt.% or from 1 wt. % to 5 wt. %. Additional exemplary ranges for thecomponents of the extract stream(s) and the raffinate stream(s) arelisted below in the discussion of the separation scheme.

In one embodiment, the extract stream may be treated to remove residualextraction agent therefrom thus yielding the intermediate productstream. In one embodiment, the raffinate stream may be treated to removeresidual extraction agent therefrom thus yielding the alkylenating agentstream.

The extraction agents of the extraction agent mixture may vary widely.In one embodiment, the at least two extraction agents are selected fromthe group consisting of diisobutyl ketone, cyclohexane, toluene,isopropyl acetate, o-xylene, p-xylene, m-xylene, butyl acetate, butanol,methyl acetate, methyl acrylate, diphenyl ether, ethyl acrylate,isopropyl acetate, ethyl propionate, hexane, benzene, diisopropyl ether,n,n-dimethyl aniline, dibutyl ether, tetralin, butyl acrylate,2-ethylhexyl alcohol, isophorone, ditolyl ether, dimethyl phthalate, 3,3trimethyl-cyclohexanone, biphenyl, o-dichlorobenzene, and mixturesthereof. In one embodiment, the at least two extraction agents areselected from the group consisting of ethers and alcohols. Preferably,the at least two extraction agents comprise diisobutyl ketone andcyclohexane.

In some embodiments, the alkylenating split is performed such that alower amount of acetic acid is present in the resulting extract streamand/or in the resultant intermediate product stream. Preferably, theextract stream and/or the intermediate product stream comprise little orno acetic acid. As an example, the intermediate product stream, in someembodiments, comprises less than 90 wt. % acetic acid, e.g., less than80 wt. %, less than 70 wt. %, less than 60 wt. %, or less than 50 wt. %.Surprisingly and unexpectedly, the present invention provides for thelower amounts of acetic acid in these streams, which, beneficiallyreduces or eliminates the need for further treatment thereof to removeacetic acid. In some embodiments, the intermediate product stream may betreated to remove water therefrom, e.g., to purge water.

In some embodiments, the alkylenating agent split is performed in atleast one column, e.g., at least two columns or at least three columns.Preferably, the alkylenating agent split is performed via extraction,e.g., via contact with the extraction agent mixture. In someembodiments, other separation methods, may be employed in combinationwith the extraction. For example, using precipitation methods, e.g.,crystallization, and/or azeotropic distillation may also be employedwith the extraction. Of course, other suitable separation methods may beemployed in combination with the extraction.

As noted above, the extract stream may be treated to remove extractionagent therefrom thus yielding the intermediate product stream. Theintermediate product stream comprises acrylate products. In oneembodiment, the intermediate product stream comprises a significantportion of acrylate products, e.g., acrylic acid. For example, theintermediate product stream may comprise at least 5 wt. % acrylateproducts, e.g., at least 25 wt. %, at least 40 wt. %, at least 50 wt. %,or at least 60 wt. %. In terms of ranges, the intermediate productstream may comprise from 5 wt. % to 99 wt. % acrylate products, e.g.from 10 wt. % to 90 wt. %, from 25 wt. % to 75 wt. %, or from 35 wt. %to 65 wt. %. The intermediate product stream, in one embodiment,comprises little if any alkylenating agent. For example, theintermediate product stream may comprise less than 1 wt. % alkylenatingagent, e.g., less than 0.1 wt. % alkylenating agent, less than 0.05 wt.%, or less than 0.01 wt. %. In addition to the acrylate products, theintermediate product stream optionally comprises acetic acid, water,propionic acid and other components.

In some cases, the intermediate acrylate product stream comprises higheramounts of alkylenating agent. For example, in one embodiment, theintermediate acrylate product stream comprises from 1 wt. % to 50 wt. %alkylenating agent, e.g., from 1 wt. % to 10 wt. % or from 5 wt. % to 50wt. %. In terms of limits, the intermediate acrylate product stream maycomprise at least 1 wt. % alkylenating agent, e.g., at least 5 wt. % orat least 10 wt. %.

In one embodiment, the crude product stream is optionally treated, e.g.separated, prior to the separation of alkylenating agent therefrom. Insuch cases, the treatment(s) occurs before the alkylenating agent splitis performed. In other embodiments, at least a portion of theintermediate acrylate product stream may be further treated after thealkylenating agent split. As one example, the crude product stream maybe treated to remove light ends therefrom. This treatment may occureither before or after the alkylenating agent split, preferably beforethe alkylenating agent split. In some of these cases, the furthertreatment of the intermediate acrylate product stream may result inderivative streams that may be considered to be additional purifiedacrylate product streams. In other embodiments, the further treatment ofthe intermediate acrylate product stream results in at least onefinished acrylate product stream.

In one embodiment, the inventive process operates at a high processefficiency. For example, the process efficiency may be at least 10%,e.g., at least 20% or at least 35%. In one embodiment, the processefficiency is calculated based on the flows of reactants into thereaction zone. The process efficiency may be calculated by the followingformula.

Process Efficiency=2N_(HAcA)/[N_(HOAc)+N_(HCHO)+N_(H) _(H20)]

where:

N_(HAcA) is the molar production rate of acrylate products; and

N_(HOAc), N_(HCHO), and N_(H2O) are the molar feed rates of acetic acid,formaldehyde, and water.

Production of Acrylate Products

Any suitable reaction and/or separation scheme may be employed to formthe crude product stream as long as the reaction provides the crudeproduct stream components that are discussed above. For example, in someembodiments, the acrylate product stream is formed by contacting analkanoic acid, e.g., acetic acid, or an ester thereof with analkylenating agent, e.g., a methylenating agent, for exampleformaldehyde, under conditions effective to form the crude acrylateproduct stream. Preferably, the contacting is performed over a suitablecatalyst. The crude product stream may be the reaction product of thealkanoic acid-alkylenating agent reaction. In a preferred embodiment,the crude product stream is the reaction product of the aldolcondensation reaction of acetic acid and formaldehyde, which isconducted over a catalyst comprising vanadium and titanium. In oneembodiment, the crude product stream is the product of a reaction inwherein methanol with acetic acid are combined to generate formaldehydein situ. The aldol condensation then follows. In one embodiment, amethanol-formaldehyde solution is reacted with acetic acid to form thecrude product stream.

The alkanoic acid, or an ester of the alkanoic acid, may be of theformula R—CH₂—COOR, where R and R′ are each, independently, hydrogen ora saturated or unsaturated alkyl or aryl group. As an example, R and R′may be a lower alkyl group containing for example 1-4 carbon atoms. Inone embodiment, an alkanoic acid anhydride may be used as the source ofthe alkanoic acid. In one embodiment, the reaction is conducted in thepresence of an alcohol, preferably the alcohol that corresponds to thedesired ester, e.g., methanol. In addition to reactions used in theproduction of acrylic acid, the inventive catalyst, in otherembodiments, may be employed to catalyze other reactions.

The alkanoic acid, e.g., acetic acid, may be derived from any suitablesource including natural gas, petroleum, coal, biomass, and so forth. Asexamples, acetic acid may be produced via methanol carbonylation,acetaldehyde oxidation, ethylene oxidation, oxidative fermentation, andanaerobic fermentation. As petroleum and natural gas prices fluctuate,becoming either more or less expensive, methods for producing aceticacid and intermediates such as methanol and carbon monoxide fromalternate carbon sources have drawn increasing interest. In particular,when petroleum is relatively expensive compared to natural gas, it maybecome advantageous to produce acetic acid from synthesis gas (“syngas”)that is derived from any available carbon source. U.S. Pat. No.6,232,352, which is hereby incorporated by reference, for example,teaches a method of retrofitting a methanol plant for the manufacture ofacetic acid. By retrofitting a methanol plant, the large capital costsassociated with carbon monoxide generation for a new acetic acid plantare significantly reduced or largely eliminated. All or part of thesyngas is diverted from the methanol synthesis loop and supplied to aseparator unit to recover carbon monoxide and hydrogen, which are thenused to produce acetic acid.

Methanol carbonylation processes suitable for production of acetic acidare described in U.S. Pat. Nos. 7,208,624, 7,115,772, 7,005,541,6,657,078, 6,627,770, 6,143,930, 5,599,976, 5,144,068, 5,026,908,5,001,259, and 4,994,608, all of which are hereby incorporated byreference.

U.S. Pat. No. RE 35,377, which is hereby incorporated by reference,provides a method for the production of methanol by conversion ofcarbonaceous materials such as oil, coal, natural gas and biomassmaterials. The process includes hydrogasification of solid and/or liquidcarbonaceous materials to obtain a process gas which is steam pyrolizedwith additional natural gas to form syngas. The syngas is converted tomethanol which may be carbonylated to acetic acid. U.S. Pat. No.5,821,111, which discloses a process for converting waste biomassthrough gasification into syngas, as well as U.S. Pat. No. 6,685,754 arehereby incorporated by reference.

In one optional embodiment, the acetic acid that is utilized in thecondensation reaction comprises acetic acid and may also comprise othercarboxylic acids, e.g., propionic acid, esters, and anhydrides, as wellas acetaldehyde and acetone. In one embodiment, the acetic acid fed tothe condensation reaction comprises propionic acid. For example, theacetic acid fed to the reaction may comprise from 0.001 wt. % to 15 wt.% propionic acid, e.g., from 0.001 wt. % to 0.11 wt. %, from 0.125 wt. %to 12.5 wt. %, from 1.25 wt. % to 11.25 wt. %, or from 3.75 wt. % to8.75 wt. %. Thus, the acetic acid feed stream may be a cruder aceticacid feed stream, e.g., a less-refined acetic acid feed stream.

As used herein, “alkylenating agent” means an aldehyde or precursor toan aldehyde suitable for reacting with the alkanoic acid, e.g., aceticacid, to form an unsaturated acid, e.g., acrylic acid, or an alkylacrylate. In preferred embodiments, the alkylenating agent comprises amethylenating agent such as formaldehyde, which preferably is capable ofadding a methylene group (═CH₂) to the organic acid. Other alkylenatingagents may include, for example, acetaldehyde, propanal, butanal, arylaldehydes, benzyl aldehydes, alcohols, and combinations thereof. Thislisting is not exclusive and is not meant to limit the scope of theinvention. In one embodiment, an alcohol may serve as a source of thealkylenating agent. For example, the alcohol may be reacted in situ toform the alkylenating agent, e.g., the aldehyde.

The alkylenating agent, e.g., formaldehyde, may be derived from anysuitable source. Exemplary sources may include, for example, aqueousformaldehyde solutions, anhydrous formaldehyde derived from aformaldehyde drying procedure, trioxane, diether of methylene glycol,and paraformaldehyde. In a preferred embodiment, the formaldehyde isproduced via a methanol oxidation process, which reacts methanol andoxygen to yield the formaldehyde.

In other embodiments, the alkylenating agent is a compound that is asource of formaldehyde. Where forms of formaldehyde that are not asfreely or weakly complexed are used, the formaldehyde will form in situin the condensation reactor or in a separate reactor prior to thecondensation reactor. Thus for example, trioxane may be decomposed overan inert material or in an empty tube at temperatures over 350° C. orover an acid catalyst at over 100° C. to form the formaldehyde.

In one embodiment, the alkylenating agent corresponds to Formula I.

In this formula, R₅ and R₆ may be independently selected from C₁-C₁₂hydrocarbons, preferably, C₁-C₁₂ alkyl, alkenyl or aryl, or hydrogen.Preferably, R₅ and R₆ are independently C₁-C₆ alkyl or hydrogen, withmethyl and/or hydrogen being most preferred. X may be either oxygen orsulfur, preferably oxygen; and n is an integer from 1 to 10, preferably1 to 3. In some embodiments, m is 1 or 2, preferably 1.

In one embodiment, the compound of formula I may be the product of anequilibrium reaction between formaldehyde and methanol in the presenceof water. In such a case, the compound of formula I may be a suitableformaldehyde source. In one embodiment, the formaldehyde source includesany equilibrium composition. Examples of formaldehyde sources includebut are not restricted to methylal (1,1 dimethoxymethane);polyoxymethylenes —(CH₂—O)_(i)— wherein i is from 1 to 100; formalin;and other equilibrium compositions such as a mixture of formaldehyde,methanol, and methyl propionate. In one embodiment, the source offormaldehyde is selected from the group consisting of 1,1dimethoxymethane; higher formals of formaldehyde and methanol; andCH₃—O—(CH₂—O)_(i)—CH₃ where i is 2.

The alkylenating agent may be used with or without an organic orinorganic solvent.

The term “formalin,” refers to a mixture of formaldehyde, methanol, andwater. In one embodiment, formalin comprises from 25 wt. % to 65 wt. %formaldehyde; from 0.01 wt. % to 25 wt. % methanol; and from 25 wt. % to70 wt. % water. In cases where a mixture of formaldehyde, methanol, andmethyl propionate is used, the mixture comprises less than 10 wt. %water, e.g., less than 5 wt. % or less than 1 wt. %.

In some embodiments, the condensation reaction may achieve favorableconversion of acetic acid and favorable selectivity and productivity toacrylates. For purposes of the present invention, the term “conversion”refers to the amount of acetic acid in the feed that is converted to acompound other than acetic acid. Conversion is expressed as a percentagebased on acetic acid in the feed. The conversion of acetic acid may beat least 10%, e.g., at least 20%, at least 40%, or at least 50%.

Selectivity, as it refers to the formation of acrylate product, isexpressed as the ratio of the amount of carbon in the desired product(s)and the amount of carbon in the total products. This ratio may bemultiplied by 100 to arrive at the selectivity. Preferably, the catalystselectivity to acrylate products, e.g., acrylic acid and methylacrylate, is at least 40 mol %, e.g., at least 50 mol %, at least 60 mol%, or at least 70 mol %. In some embodiments, the selectivity to acrylicacid is at least 30 mol %, e.g., at least 40 mol %, or at least 50 mol%; and/or the selectivity to methyl acrylate is at least 10 mol %, e.g.,at least 15 mol %, or at least 20 mol %.

The terms “productivity” or “space time yield” as used herein, refers tothe grams of a specified product, e.g., acrylate products, formed perhour during the condensation based on the liters of catalyst used. Aproductivity of at least 20 grams of acrylate product per liter catalystper hour, e.g., at least 40 grams of acrylates per liter catalyst perhour or at least 100 grams of acrylates per liter catalyst per hour, ispreferred. In terms of ranges, the productivity preferably is from 20 to500 grams of acrylates per liter catalyst per hour, e.g., from 20 to 200per kilogram catalyst per hour or from 40 to 140 per kilogram catalystper hour.

In one embodiment, the inventive process yields at least 1,800 kg/hr offinished acrylic acid, e.g., at least 3,500 kg/hr, at least 18,000kg/hr, or at least 37,000 kg/hr.

Preferred embodiments of the inventive process demonstrate a lowselectivity to undesirable products, such as carbon monoxide and carbondioxide. The selectivity to these undesirable products preferably isless than 29%, e.g., less than 25% or less than 15%. More preferably,these undesirable products are not detectable. Formation of alkanes,e.g., ethane, may be low, and ideally less than 2%, less than 1%, orless than 0.5% of the acetic acid passed over the catalyst is convertedto alkanes, which have little value other than as fuel.

The alkanoic acid or ester thereof and alkylenating agent may be fedindependently or after prior mixing to a reactor containing thecatalyst. The reactor may be any suitable reactor or combination ofreactors. Preferably, the reactor comprises a fixed bed reactor or aseries of fixed bed reactors. In one embodiment, the reactor is a packedbed reactor or a series of packed bed reactors. In one embodiment, thereactor is a fixed bed reactor. Of course, other reactors such as acontinuous stirred tank reactor or a fluidized bed reactor, may beemployed.

In some embodiments, the alkanoic acid, e.g., acetic acid, and thealkylenating agent, e.g., formaldehyde, are fed to the reactor at amolar ratio of at least 0.10:1, e.g., at least 0.75:1 or at least 1:1.In terms of ranges the molar ratio of alkanoic acid to alkylenatingagent may range from 0.10:1 to 10:1 or from 0.75:1 to 5:1. In someembodiments, the reaction of the alkanoic acid and the alkylenatingagent is conducted with a stoichiometric excess of alkanoic acid. Inthese instances, acrylate selectivity may be improved. As an example theacrylate selectivity may be at least 10% higher than a selectivityachieved when the reaction is conducted with an excess of alkylenatingagent, e.g., at least 20% higher or at least 30% higher. In otherembodiments, the reaction of the alkanoic acid and the alkylenatingagent is conducted with a stoichiometric excess of alkylenating agent.

The condensation reaction may be conducted at a temperature of at least250° C., e.g., at least 300° C., or at least 350° C. In terms of ranges,the reaction temperature may range from 200° C. to 500° C., e.g., from250° C. to 400° C., or from 250° C. to 350° C. Residence time in thereactor may range from 1 second to 200 seconds, e.g., from 1 second to100 seconds. Reaction pressure is not particularly limited, and thereaction is typically performed near atmospheric pressure. In oneembodiment, the reaction may be conducted at a pressure ranging from 0kPa to 4100 kPa, e.g., from 3 kPa to 345 kPa, or from 6 kPa to 103 kPa.The acetic acid conversion, in some embodiments, may vary depending uponthe reaction temperature.

In one embodiment, the reaction is conducted at a gas hourly spacevelocity (“GHSV”) greater than 600 hr⁻¹, e.g., greater than 1000 hr⁻¹ orgreater than 2000 hr⁻¹. In one embodiment, the GHSV ranges from 600 hr⁻¹to 10000 hr⁻¹, e.g., from 1000 hr⁻¹ to 8000 hr⁻¹ or from 1500 hr⁻¹ to7500 hr⁻¹. As one particular example, when GHSV is at least 2000 hr⁻¹,the acrylate product STY may be at least 150 g/hr/liter.

Water may be present in the reactor in amounts up to 60 wt. %, by weightof the reaction mixture, e.g., up to 50 wt. % or up to 40 wt. %. Water,however, is preferably reduced due to its negative effect on processrates and separation costs.

In one embodiment, an inert or reactive gas is supplied to the reactantstream. Examples of inert gases include, but are not limited to,nitrogen, helium, argon, and methane. Examples of reactive gases orvapors include, but are not limited to, oxygen, carbon oxides, sulfuroxides, and alkyl halides. When reactive gases such as oxygen are addedto the reactor, these gases, in some embodiments, may be added in stagesthroughout the catalyst bed at desired levels as well as feeding withthe other feed components at the beginning of the reactors. The additionof these additional components may improve reaction efficiencies.

In one embodiment, the unreacted components such as the alkanoic acidand formaldehyde as well as the inert or reactive gases that remain arerecycled to the reactor after sufficient separation from the desiredproduct.

When the desired product is an unsaturated ester made by reacting anester of an alkanoic acid ester with formaldehyde, the alcoholcorresponding to the ester may also be fed to the reactor either with orseparately to the other components. For example, when methyl acrylate isdesired, methanol may be fed to the reactor. The alcohol, amongst othereffects, reduces the quantity of acids leaving the reactor. It is notnecessary that the alcohol is added at the beginning of the reactor andit may for instance be added in the middle or near the back, in order toeffect the conversion of acids such as propionic acid, methacrylic acidto their respective esters without depressing catalyst activity. In oneembodiment, the alcohol may be added downstream of the reactor.

Catalyst Composition

The catalyst may be any suitable catalyst composition. As one example,condensation catalyst consisting of mixed oxides of vanadium andphosphorus have been investigated and described in M. Ai, J. Catal.,107, 201 (1987); M. Ai, J. Catal., 124, 293 (1990); M. Ai, Appl. Catal.,36, 221 (1988); and M. Ai, Shokubai, 29, 522 (1987). Other examplesinclude binary vanadium-titanium phosphates, vanadium-silica-phosphates,and alkali metal-promoted silicas, e.g., cesium- or potassium-promotedsilicas.

In a preferred embodiment, the inventive process employs a catalystcomposition comprising vanadium, titanium, and optionally at least oneoxide additive. The oxide additive(s), if present, are preferablypresent in the active phase of the catalyst. In one embodiment, theoxide additive(s) are selected from the group consisting of silica,alumina, zirconia, and mixtures thereof or any other metal oxide otherthan metal oxides of titanium or vanadium. Preferably, the molar ratioof oxide additive to titanium in the active phase of the catalystcomposition is greater than 0.05:1, e.g., greater than 0.1:1, greaterthan 0.5:1, or greater than 1:1. In terms of ranges, the molar ratio ofoxide additive to titanium in the inventive catalyst may range from0.05:1 to 20:1, e.g., from 0.1:1 to 10:1, or from 1:1 to 10:1. In theseembodiments, the catalyst comprises titanium, vanadium, and one or moreoxide additives and has relatively high molar ratios of oxide additiveto titanium.

In other embodiments, the catalyst may further comprise other compoundsor elements (metals and/or non-metals). For example, the catalyst mayfurther comprise phosphorus and/or oxygen. In these cases, the catalystmay comprise from 15 wt. % to 45 wt. % phosphorus, e.g., from 20 wt. %to 35 wt. % or from 23 wt. % to 27 wt. %; and/or from 30 wt. % to 75 wt.% oxygen, e.g., from 35 wt. % to 65 wt. % or from 48 wt. % to 51 wt. %.

In some embodiments, the catalyst further comprises additional metalsand/or oxide additives. These additional metals and/or oxide additivesmay function as promoters. If present, the additional metals and/oroxide additives may be selected from the group consisting of copper,molybdenum, tungsten, nickel, niobium, and combinations thereof. Otherexemplary promoters that may be included in the catalyst of theinvention include lithium, sodium, magnesium, aluminum, chromium,manganese, iron, cobalt, calcium, yttrium, ruthenium, silver, tin,barium, lanthanum, the rare earth metals, hafnium, tantalum, rhenium,thorium, bismuth, antimony, germanium, zirconium, uranium, cesium, zinc,and silicon and mixtures thereof. Other modifiers include boron,gallium, arsenic, sulfur, halides, Lewis acids such as BF₃, ZnBr₂, andSnCl₄. Exemplary processes for incorporating promoters into catalyst aredescribed in U.S. Pat. No. 5,364,824, the entirety of which isincorporated herein by reference.

If the catalyst comprises additional metal(s) and/or metal oxides(s),the catalyst optionally may comprise additional metals and/or metaloxides in an amount from 0.001 wt. % to 30 wt. %, e.g., from 0.01 wt. %to 5 wt. % or from 0.1 wt. % to 5 wt. %. If present, the promoters mayenable the catalyst to have a weight/weight space time yield of at least25 grams of acrylic acid/gram catalyst-h, e.g., least 50 grams ofacrylic acid/gram catalyst-h, or at least 100 grams of acrylic acid/gramcatalyst-h.

In some embodiments, the catalyst is unsupported. In these cases, thecatalyst may comprise a homogeneous mixture or a heterogeneous mixtureas described above. In one embodiment, the homogeneous mixture is theproduct of an intimate mixture of vanadium and titanium oxides,hydroxides, and phosphates resulting from preparative methods such ascontrolled hydrolysis of metal alkoxides or metal complexes. In otherembodiments, the heterogeneous mixture is the product of a physicalmixture of the vanadium and titanium phosphates. These mixtures mayinclude formulations prepared from phosphorylating a physical mixture ofpreformed hydrous metal oxides. In other cases, the mixture(s) mayinclude a mixture of preformed vanadium pyrophosphate and titaniumpyrophosphate powders.

In another embodiment, the catalyst is a supported catalyst comprising acatalyst support in addition to the vanadium, titanium, oxide additive,and optionally phosphorous and oxygen, in the amounts indicated above(wherein the molar ranges indicated are without regard to the moles ofcatalyst support, including any vanadium, titanium, oxide additive,phosphorous or oxygen contained in the catalyst support). The totalweight of the support (or modified support), based on the total weightof the catalyst, preferably is from 75 wt. % to 99.9 wt. %, e.g., from78 wt. % to 97 wt. % or from 80 wt. % to 95 wt. %. The support may varywidely. In one embodiment, the support material is selected from thegroup consisting of silica, alumina, zirconia, titania,aluminosilicates, zeolitic materials, mixed metal oxides (including butnot limited to binary oxides such as SiO₂—Al₂O₃, SiO₂—TiO₂, SiO₂—ZnO,SiO₂—MgO, SiO₂—ZrO₂, Al₂O₃—MgO, Al₂O₃—TiO₂, Al₂O₃—ZnO, TiO₂—MgO,TiO₂—ZrO₂, TiO₂—ZnO, TiO₂—SnO₂) and mixtures thereof, with silica beingone preferred support. In embodiments where the catalyst comprises atitania support, the titania support may comprise a major or minoramount of rutile and/or anatase titanium dioxide. Other suitable supportmaterials may include, for example, stable metal oxide-based supports orceramic-based supports. Preferred supports include silicaceous supports,such as silica, silica/alumina, a Group IIA silicate such as calciummetasilicate, pyrogenic silica, high purity silica, silicon carbide,sheet silicates or clay minerals such as montmorillonite, beidellite,saponite, pillared clays, other microporous and mesoporous materials,and mixtures thereof. Other supports may include, but are not limitedto, iron oxide, magnesia, steatite, magnesium oxide, carbon, graphite,high surface area graphitized carbon, activated carbons, and mixturesthereof. These listings of supports are merely exemplary and are notmeant to limit the scope of the present invention.

In some embodiments, a zeolitic support is employed. For example, thezeolitic support may be selected from the group consisting ofmontmorillonite, NH₄ ferrierite, H-mordenite-PVOx, vermiculite-1,H-ZSM5, NaY, H-SDUSY, Y zeolite with high SAR, activated bentonite,H-USY, MONT-2, HY, mordenite SAR 20, SAPO-34, Aluminosilicate (X), VUSY,Aluminosilicate (CaX), Re—Y, and mixtures thereof. H-SDUSY, VUSY, andH-USY are modified Y zeolites belonging to the faujasite family. In oneembodiment, the support is a zeolite that does not contain any metaloxide modifier(s). In some embodiments, the catalyst compositioncomprises a zeolitic support and the active phase comprises a metalselected from the group consisting of vanadium, aluminum, nickel,molybdenum, cobalt, iron, tungsten, zinc, copper, titanium cesiumbismuth, sodium, calcium, chromium, cadmium, zirconium, and mixturesthereof. In some of these embodiments, the active phase may alsocomprise hydrogen, oxygen, and/or phosphorus.

In other embodiments, in addition to the active phase and a support, theinventive catalyst may further comprise a support modifier. A modifiedsupport, in one embodiment, relates to a support that includes a supportmaterial and a support modifier, which, for example, may adjust thechemical or physical properties of the support material such as theacidity or basicity of the support material. In embodiments that use amodified support, the support modifier is present in an amount from 0.1wt. % to 50 wt. %, e.g., from 0.2 wt. % to 25 wt. %, from 0.5 wt. % to15 wt. %, or from 1 wt. % to 8 wt. %, based on the total weight of thecatalyst composition.

In one embodiment, the support modifier is an acidic support modifier.In some embodiments, the catalyst support is modified with an acidicsupport modifier. The support modifier similarly may be an acidicmodifier that has a low volatility or little volatility. The acidicmodifiers may be selected from the group consisting of oxides of GroupIVB metals, oxides of Group VB metals, oxides of Group VIB metals, ironoxides, aluminum oxides, and mixtures thereof. In one embodiment, theacidic modifier may be selected from the group consisting of WO₃, MoO₃,Fe₂O₃, Cr₂O₃, V₂O₅, MnO₂, CuO, Co₂O₃, Bi₂O₃, TiO₂, ZrO₂, Nb₂O₅, Ta₂O₅,Al₂O₃, B₂O₃, P₂O₅, and Sb₂O₃.

In another embodiment, the support modifier is a basic support modifier.The presence of chemical species such as alkali and alkaline earthmetals, are normally considered basic and may conventionally beconsidered detrimental to catalyst performance. The presence of thesespecies, however, surprisingly and unexpectedly, may be beneficial tothe catalyst performance. In some embodiments, these species may act ascatalyst promoters or a necessary part of the acidic catalyst structuresuch in layered or sheet silicates such as montmorillonite. Withoutbeing bound by theory, it is postulated that these cations create astrong dipole with species that create acidity.

Additional modifiers that may be included in the catalyst include, forexample, boron, aluminum, magnesium, zirconium, and hafnium.

As will be appreciated by those of ordinary skill in the art, thesupport materials, if included in the catalyst of the present invention,preferably are selected such that the catalyst system is suitablyactive, selective and robust under the process conditions employed forthe formation of the desired product, e.g., acrylic acid or alkylacrylate. Also, the active metals and/or pyrophosphates that areincluded in the catalyst of the invention may be dispersed throughoutthe support, coated on the outer surface of the support (egg shell) ordecorated on the surface of the support. In some embodiments, in thecase of macro- and meso-porous materials, the active sites may beanchored or applied to the surfaces of the pores that are distributedthroughout the particle and hence are surface sites available to thereactants but are distributed throughout the support particle.

The inventive catalyst may further comprise other additives, examples ofwhich may include: molding assistants for enhancing moldability;reinforcements for enhancing the strength of the catalyst; pore-formingor pore modification agents for formation of appropriate pores in thecatalyst, and binders. Examples of these other additives include stearicacid, graphite, starch, cellulose, silica, alumina, glass fibers,silicon carbide, and silicon nitride. Preferably, these additives do nothave detrimental effects on the catalytic performances, e.g., conversionand/or activity. These various additives may be added in such an amountthat the physical strength of the catalyst does not readily deteriorateto such an extent that it becomes impossible to use the catalystpractically as an industrial catalyst.

Separation of Acrylic Acid and Formaldehyde

As discussed above, the crude product stream is separated to yield anintermediate acrylate product stream. FIG. 1 is a flow diagram depictingthe formation of the crude product stream and the separation thereof toobtain a stream comprising separated acrylate product. Acrylate productsystem 100 comprises reaction zone 102 and separation zone 103. Reactionzone 102 comprises reactor 106, alkanoic acid feed, e.g., acetic acidfeed, 108, alkylenating agent feed, e.g., formaldehyde feed 110, andvaporizer 112.

Acetic acid and formaldehyde are fed to vaporizer 112 via lines 108 and110, respectively, to create a vapor feed stream, which exits vaporizer112 via line 114 and is directed to reactor 106. In one embodiment,lines 108 and 110 may be combined and jointly fed to the vaporizer 112.The temperature of the vapor feed stream in line 114 is preferably from200° C. to 600° C., e.g., from 250° C. to 500° C. or from 340° C. to425° C. Alternatively, a vaporizer may not be employed and the reactantsmay be fed directly to reactor 106.

Any feed that is not vaporized may be removed from vaporizer 112 and maybe recycled or discarded. In addition, although line 114 is shown asbeing directed to the upper half of reactor 106, line 114 may bedirected to the middle or bottom of first reactor 106. Furthermodifications and additional components to reaction zone 102 andseparation zone 104 are described below.

Reactor 106 contains the catalyst that is used in the reaction to formcrude product stream, which is withdrawn, preferably continuously, fromreactor 106 via line 116. Although FIG. 1 shows the crude product streambeing withdrawn from the bottom of reactor 106, the crude product streammay be withdrawn from any portion of reactor 106. Exemplary compositionranges for the crude product stream are shown in Table 1 above.

In one embodiment, one or more guard beds (not shown) may be usedupstream of the reactor to protect the catalyst from poisons orundesirable impurities contained in the feed or return/recycle streams.Such guard beds may be employed in the vapor or liquid streams. Suitableguard bed materials may include, for example, carbon, silica, alumina,ceramic, or resins. In one aspect, the guard bed media isfunctionalized, e.g., silver functionalized, to trap particular speciessuch as sulfur or halogens.

The crude product stream in line 116 is fed to separation zone 103,which comprises alkylenating agent split unit 104. Although onlyalkylenating agent split unit 104 is shown in FIG. 1, separation zone103 of FIG. 1 may further comprise additional separation units, asdiscussed herein. Alkylenating agent split unit 104 may comprise one ormore separation units, e.g., two or more or three or more. In oneexample, alkylenating agent split unit 104 contains multiple columns,e.g., extraction columns, as shown in FIG. 2. Alkylenating agent splitunit 104 separates the crude product stream into at least oneintermediate acrylate product stream, which exits via line 118 and atleast one alkylenating agent stream, which exits via line 120. Exemplarycompositional ranges for the intermediate acrylate product stream areshown in Table 2. Components other than those listed in Table 2 may alsobe present in the intermediate acrylate product stream. Examples includemethanol, methyl acetate, methyl acrylate, dimethyl ketone, carbondioxide, carbon monoxide, oxygen, nitrogen, and acetone.

TABLE 2 INTERMEDIATE ACRYLATE PRODUCT STREAM COMPOSITION Conc. (wt. %)Conc. (wt. %) Conc. (wt. %) Acrylic Acid at least 5 5 to 99 35 to 65Acetic Acid less than 95 5 to 90 20 to 60 Water less than 25 0.1 to 10  0.5 to 7   Alkylenating Agent  <1 <0.5 <0.1 Propionic Acid <10 0.01 to5    0.01 to 1  

In other embodiments, the intermediate acrylate product stream compriseshigher amounts of alkylenating agent. For example, the intermediateacrylate product stream may comprise from 1 wt. % to 10 wt. %alkylenating agent, e.g., from 1 wt. % to 8 wt. % or from 2 wt. % to 5wt. %. In one embodiment, the intermediate acrylate product streamcomprises greater than 1 wt. % alkylenating agent, e.g., greater than 5wt. % or greater than 10 wt. %.

Exemplary compositional ranges for the alkylenating stream are shown inTable 3. Components other than those listed in Table 3 may also bepresent in the purified alkylate product stream. Examples includemethanol, methyl acetate, methyl acrylate, dimethyl ketone, carbondioxide, carbon monoxide, oxygen, nitrogen, and acetone.

TABLE 3 ALKYLENATING STREAM COMPOSITION Conc. (wt. %) Conc. (wt. %)Conc. (wt. %) Acrylic Acid less than 15 0.01 to 10   0.1 to 5   AceticAcid 10 to 65 20 to 65 25 to 55 Water 15 to 75 25 to 65 30 to 60Alkylenating Agent at least 1  1 to 75 10 to 20 Propionic Acid <10 0.001to 5    0.001 to 1   

In other embodiments, the alkylenating stream comprises lower amounts ofacetic acid. For example, the alkylenating agent stream may compriseless than 10 wt. % acetic acid, e.g., less than 5 wt. % or less than 1wt. %.

As mentioned above, the crude product stream of the present inventioncomprises little, if any, furfural and/or acrolein. As such thederivative stream(s) of the crude product streams will comprise little,if any, furfural and/or acrolein. In one embodiment, the derivativestream(s), e.g., the streams of the separation zone, comprises less thanless than 500 wppm acrolein, e.g., less than 100 wppm, less than 50wppm, or less than 10 wppm. In one embodiment, the derivative stream(s)comprises less than less than 500 wppm furfural, e.g., less than 100wppm, less than 50 wppm, or less than 10 wppm.

FIG. 2 shows an overview of a portion of a reaction/separation scheme inaccordance with the present invention. Acrylate product system 200comprises reaction zone 202 and separation zone 203. Separation zone 203comprises alkylenating agent split unit 204, which performs thealkylenating split of the separation process. Reaction zone 202comprises reactor 206, alkanoic acid feed, e.g., acetic acid feed, 208,alkylenating agent feed, e.g., formaldehyde feed, 210, vaporizer 212,and line 214. Reaction zone 202 and the components thereof function in amanner similar to reaction zone 102 of FIG. 1.

Reaction zone 202 yields a crude product stream, which exits reactionzone 202 via line 216 and is directed to separation zone 203. Thecomponents of the crude product stream are discussed above. In additionto alkylenating agent split unit 204, the separation zone may furthercomprise an acetic acid split unit and a drying unit. Alkylenating splitunit 204 may also comprise an optional light ends removal unit (notshown). For example, the light ends removal unit may comprise acondenser and/or a flasher. The light ends removal unit may beconfigured either upstream or downstream of the alkylenating agent splitunit. Depending on the configuration, the light ends removal unitremoves light ends from the crude product stream, the alkylenatingstream, and/or the intermediate acrylate product stream. In oneembodiment, when the light ends are removed, the remaining liquid phasecomprises the acrylic acid, acetic acid, alkylenating agent, and/orwater.

As shown in FIG. 2, alkylenating agent split unit 204 comprisesextraction column 218, and extraction agent recovery columns 220, 222,and 237. Alkylenating agent split unit 204 receives crude acrylicproduct stream in line 216 and separates same into at least one streamcomprising alkylenating agent, and at least one stream comprisingacrylate product. In accordance with an embodiment of the invention, thecrude acrylic product is fed to liquid-liquid extraction column 218.Extraction column 218 utilizes the inventive extraction agent mixture toeffectively extract acrylic acid to the (organic) extract stream 226 andto form the (aqueous) raffinate stream 224. The extract stream comprisesacrylate product and extractive agents and the raffinate streamcomprises water, formaldehyde, some acetic acid and a small amount ofthe extractive agents.

As shown in FIG. 2, crude product stream 216 is fed to liquid-liquidextraction column 218 where the crude product stream is contacted withtwo or more extraction agents, e.g., organic solvents, which are fed vialine 228. Although one extraction agent feed line is shown, multipleextraction agent feed lines may also be employed. Liquid-liquidextraction column 218 extracts the acrylate products, e.g., acrylicacid, from crude product stream 216 into extract stream 226. The extractstream further comprises extraction agents, e.g., organic solvent(s).Acrylate products may be separated from extract stream 226 and collectedas a first intermediate acrylate product stream in line 230. Solvent(s)may be separated from raffinate stream 224 and re-used or recycled.Thus, in one embodiment, the process further comprises the step ofseparating the extract stream to form an intermediate product stream anda first solvent stream. The organic solvent(s) may be separated andrecycled to liquid-liquid extraction unit 218 via line 232. Treatment ofthe extract stream and derivatives thereof is discussed below.

In one embodiment, the organic extract stream is substantially free ofwater and formaldehyde. In an embodiment, the extraction is carried outat a temperature such that the organic extract stream is substantiallyfree of water and formaldehyde.

Raffinate stream 224 comprises water, alkylenating agent, acetic acid,and organic solvent(s) and exits liquid-liquid extraction unit 218.Alkylenating agent may be separated from raffinate stream 224 andcollected as alkylenating agent stream in line 236. Solvent(s) may beseparated from raffinate stream 224 and re-used or recycled. Thus, inone embodiment, the process further comprises the step of separating theraffinate stream to form an alkylenating agent stream and a secondsolvent stream comprising the organic solvent(s). The organic solvent(s)may be separated and recycled to liquid-liquid extraction unit 218 vialine 234. Treatment of the raffinate stream and derivatives thereof isdiscussed below.

At least a portion of raffinate stream 224 may be further treated and/orrecycled. For example, the acetic acid in the extract stream and theraffinate stream may be separated then recycled and/or used in this orother processes.

In one embodiment, liquid-liquid extraction column 218 may be anyconventional liquid-liquid extraction device, for example, a staticmixer, a stirred vessel, a mixer/settler, a rotary-disc extractor, anextractor with centrifugation or a column with perforated plates orpacking. In one embodiment, liquid-liquid extraction column 218 may be atray column having from 5 to 70 trays, e.g., from 15 to 50 trays, orfrom 20 to 45 trays. In one embodiment the liquid-liquid extractioncolumn may be an agitated column extractor, a pulsed column extractor,and/or a disc-and-donut column.

In one embodiment, extraction column 218 may operate counter-currently,meaning that the extraction agent mixture and the crude acrylic productstream flow in opposite directions of one another. In anotherembodiment, extraction column 218 may operate co-currently, meaning thatthe extraction agent mixture and the crude acrylic product stream flowin the same direction.

In one embodiment, the extraction may be carried out is a continuousmanner. In another embodiment, the extraction may be carried out in abatch-wise manner.

In an embodiment, the extraction agent mixture is introduced toliquid-liquid extraction column 218 via line 228, preferably in thebottom part of the column, e.g., bottom half or bottom third.

The inventors discovered that by using an extraction agent mixture thatis less volatile than acrylic acid, the energy cost is surprisinglyreduced because the extraction agent is not boiled overhead in adistillation column with the acrylic acid. Thus, in one embodiment, thesuitable extraction agent mixture is less volatile than acrylic acid. Asan additional benefit, it has been discovered that, unexpectedly, thepotential for acrylic acid to undergo polymerization is reduced when theinventive extraction agent(s) are employed. In some embodiments, thetemperature at which the extraction may be carried out depends upon theextraction agent mixture being used and the components in the crudeacrylic product stream.

In an embodiment, the extraction is carried out at a temperature lowerthan 49° C., e.g., lower than 38° C., lower than 27° C., or lower than16° C. It has now been found that the selectivity of the extractiveagent for acrylic acid versus formaldehyde is enhanced when theliquid-liquid extraction is carried out at a temperature lower thanambient temperature. To achieve these temperatures, the crude productstream may be cooled by suitable cooling units. As one example, thecrude product stream may be cooled using one or more condensers. Ofcourse other cooling units may be used along with or in place of thecondenser(s).

Exemplary compositional ranges for extract stream 226 and raffinatestream 224 of extraction column 218 are shown in Table 4. Componentsother than those listed in Table 4 may also be present in the raffinateand extract streams.

TABLE 4 EXTRACTION COLUMN Conc. (wt. %) Conc. (wt. %) Conc. (wt. %)Raffinate Stream Acrylic Acid Less than 0.1 0.1 to 5   0.5 to 3   AceticAcid 1 to 35  2 to 30  7 to 25 Water 47 to 93  50 to 78 50 to 73Alkylenating Agent 1 to 42 12 to 27 15 to 22 Extraction Agent 0.001 to5    0.01 to 1   0.01 to 0.05 Extract Stream Acrylic Acid 1 to 40  8 to25 13 to 25 Acetic Acid 1 to 40 10 to 30 15 to 25 Water 0.1 to 26    1to 15  3 to 10 Alkylenating Agent 0.01 to 23   0.1 to 8   0.5 to 3  Extraction Agent 25 to 85  35 to 70 40 to 60

In one embodiment, the crude acrylic product feed may be of higherdensity than the extraction agent mixture. In such embodiments, theextraction agent mixture may be fed to a point in the liquid-liquidextraction column below the feed point of the crude acrylic productfeed. In another embodiment, the crude acrylic product feed may be oflower density than the extraction agent. In such embodiments, theextraction agent may be fed at a point in the extraction column abovethe crude acrylic product feed.

It is noted that FIG. 2 is merely an exemplary embodiment of theliquid-liquid extractive distillation separation process. Although theorganic extract stream 226 is shown as a distillate and raffinate stream224 is shown as a residue stream, it is noted that, depending on theextractive agent used, the organic extract stream 226 may be a residuestream and the aqueous stream 226 may be a distillate stream.

Returning to the extraction agent recovery columns of FIG. 2, at least aportion of extract stream 226 may be fed to extractive agent recoverycolumn 220. Extraction agent recovery column 220 separates the at leasta portion of extract stream 226 into a first intermediate acrylateproduct stream in line 230 and a first solvent stream in line 232. Firstintermediate acrylate product stream 230 may be refluxed as shown andfirst solvent stream 232 may be boiled up as shown. First intermediateacrylate product stream 230 comprises at least 1 wt. % acrylic acid.First intermediate stream 230, like stream 226, may be considered anacrylate product stream. In one embodiment, at least a portion of thecontents of line 232 is returned, either directly or indirectly, toextraction column 218.

In an embodiment, first intermediate acrylate product stream 230comprises acrylic acid and acetic acid. In one embodiment, firstintermediate acrylate product stream 230 may comprise some remainingextraction agent. First intermediate acrylate product stream 230 may befurther processed, e.g., in an additional extraction removal column, toremove additional extraction agent.

Exemplary compositional ranges for the first intermediate acrylateproduct stream 230 and first solvent stream 232 of the first solventrecovery column 220 are shown in Table 5. Components other than thoselisted in Table 5 may also be present in the residue and distillate.

TABLE 5 SOLVENT RECOVERY COLUMN 220 Conc. (wt. %) Conc. (wt. %) Conc.(wt. %) Distillate Acrylic Acid 10 to 70  20 to 55  30 to 45 Acetic Acid10 to 70  20 to 55  30 to 45 Water 1 to 35 1 to 25  5 to 15 AlkylenatingAgent 1 to 25 1 to 15 1 to 7 Extractive Agent 1 to 20 1 to 15 less than10 Residue Acrylic Acid 0.01 to 5    0.01 to 1    less than 0.5 AceticAcid 0.01 to 15   0.1 to 10   less than 1 Water less than 0.1 less than0.01 less than 0.001 Alkylenating Agent less than 0.1 less than 0.01less than 0.001 Extractive Agent 90 to 100 95 to 100  99 to 100

In one embodiment, the first intermediate acrylate product stream 230,may be further processed to further remove extraction agent. As shown inFIG. 2, first intermediate acrylate product stream 230 is fed to solventrecovery column 237. Extraction agent recovery column 237 separates theat least a portion of first intermediate acrylate product stream 230into a second intermediate acrylate product stream in line 239 and asecond solvent stream in line 238. Second intermediate acrylate productstream 239 may be boiled up as shown and second solvent stream 238 maybe refluxed as shown. Second intermediate acrylate product stream 239comprises acrylic acid and/or acetic acid. Stream intermediate acrylateproduct stream 239 may be considered an acrylate product stream. In oneembodiment, at least a portion of the contents of line 238 is returned,either directly or indirectly, to extraction column 218.

Exemplary compositional ranges for second intermediate acrylate productstream 239 and second solvent stream 238 of second solvent recoverycolumn 237 are shown in Table 6. Components other than those listed inTable 6 may also be present in the residue and distillate.

TABLE 6 SOLVENT RECOVERY COLUMN 237 Conc. (wt. %) Conc. (wt. %) Conc.(wt. %) Distillate Acrylic Acid 0.01 to 10   0.1 to 5   0.1 to 1  Acetic Acid 0.1 to 10   0.1 to 5   1 to 3 Water 1 to 65 10 to 55 20 to40 Alkylenating Agent 0.1 to 15   0.1 to 5   0.5 to 3   Extractive Agent1 to 20  1 to 15 less than 10 Residue Acrylic Acid 1 to 85 20 to 65 30to 55 Acetic Acid 1 to 85 20 to 65 30 to 55 Water 1 to 30  2 to 20  5 to15 Alkylenating Agent 0.1 to 20    1 to 10 1 to 5 Extractive Agent lessthan 0.1 less than 0.01 less than 0.001

Returning to the raffinate of column 218, the extractive agent inraffinate 224 may be separated and recycled to extraction column 218. Asshown in FIG. 2, at least a portion of raffinate stream 224 may be fedto third extraction agent recovery column 222. Third extraction agentrecovery column 222 separates the at least a portion of raffinate stream224 into an alkylenating agent stream in line 236 and a third solventstream in line 234. Alkylenating agent stream 236 may be boiled up andthird solvent stream 234 may be refluxed as shown. Alkylenating agentstream 236, as discussed above, comprises at least 1 wt. % alkylenatingagent. In one embodiment, at least a portion of third solvent stream inline 234 is returned, either directly or indirectly, to extractioncolumn 218.

Exemplary compositional ranges for the distillate 234 and residue 226 ofthe second solvent recovery column 222 are shown in Table 7. Componentsother than those listed in Table 7 may also be present in the residueand distillate.

TABLE 7 SOLVENT RECOVERY COLUMN 222 Conc. (wt. %) Conc. (wt. %) Conc.(wt. %) Distillate Acrylic Acid less than 0.1 less than 0.01 less than0.001 Acetic Acid 0.01 to 10   0.1 to 5   0.1 to 2   Water 50 to 95 60to 85 65 to 80 Alkylenating Agent  1 to 50 10 to 40 20 to 30 ExtractiveAgent 0.01 to 10   0.1 to 5   0.1 to 3   Residue Acrylic Acid less than0.1 less than 0.01 less than 0.001 Acetic Acid  1 to 50 10 to 40 20 to30 Water 30 to 90 40 to 80 50 to 70 Alkylenating Agent  1 to 30  5 to 2510 to 20 Extractive Agent less than 0.1 less than 0.01 less than 0.001

Although columns are shown in FIG. 2, any suitable separation unit maybe employed to separate the extractive agent(s) from the respectivestreams. For example, (additional) liquid-liquid extraction,crystallization, and/or evaporation may be used to recover extractionagent(s) from organic and aqueous streams.

In cases where the alkylenating agent split unit comprises at least oneliquid-liquid extraction unit, the unit(s) may be operated at suitabletemperatures and pressures. In one embodiment, the temperature of theextract exiting the unit(s) ranges from 20° C. to 100° C., e.g., from25° C. to 80° C. or from 35° C. to 60° C. The temperature of theraffinate exiting the unit(s) preferably ranges from 20° C. to 100° C.,e.g., from 25° C. to 80° C. or from 35° C. to 60° C. The pressure atwhich the units(s) are operated may range from 1 kPa to 150 kPa, e.g.,from 10 kPa to 100 kPa or from 40 kPa to 80 kPa. In preferredembodiments, the pressure at which the unit(s) are operated is kept at alow level e.g., less than 100 kPa, less than 80 kPa, or less than 60kPa. In terms of lower limits, the units(s) may be operated at apressures of at least 1 kPa, e.g., at least 20 kPa or at least 40 kPa.Without being bound by theory, the maintenance of the column pressuresat these levels surprisingly and unexpectedly provides for efficientseparation operations.

The inventive process further comprises the step of separating theintermediate acrylate product stream 239 to form a finished acrylateproduct stream and a first finished acetic acid stream. The finishedacrylate product stream comprises acrylate product(s) and the firstfinished acetic acid stream comprises acetic acid. The separation of theacrylate products from the intermediate product stream to form thefinished acrylate product may be referred to as the “acrylate productsplit.”

The inventive process further comprises the step of separating theintermediate acrylate product stream to form a finished acrylate productstream and a first finished acetic acid stream. The finished acrylateproduct stream comprises acrylate product(s) and the first finishedacetic acid stream comprises acetic acid. The separation of the acrylateproducts from the intermediate product stream to form the finishedacrylate product may be referred to as the “acrylate product split.”

As shown in FIG. 3, intermediate product stream 239 exits alkylenatingagent split unit 204 and is directed to acrylate product split unit 240for further separation, e.g., to further separate the acrylate productstherefrom. Acrylate product split unit 240 may comprise any suitableseparation device or combination of separation devices. For example,acrylate product split unit 240 may comprise at least one column, e.g.,a standard distillation column, an extractive distillation column and/oran azeotropic distillation column. In other embodiments, acrylateproduct split unit 240 comprises a precipitation unit, e.g., acrystallizer and/or a chiller. Preferably, acrylate product split unit240 comprises two standard distillation columns as shown in FIG. 3. Inanother embodiment, acrylate product split unit 240 comprises aliquid-liquid extraction unit. Of course, other suitable separationdevices may be employed either alone or in combination with the devicesmentioned herein.

In FIG. 3, acrylate product split unit 240 comprises fourth column 246and fifth column 248. Acrylate product split unit 240 receives at leasta portion of purified acrylic product stream in line 239 and separatessame into finished acrylate product stream 256 and at least one aceticacid-containing stream. As such, acrylate product split unit 240 mayyield the finished acrylate product.

As shown in FIG. 3, at least a portion of purified acrylic productstream in line 239 is directed to fourth column 246. Fourth column 246separates the intermediate acrylate product stream to form fourthdistillate, e.g., line 254, and fourth residue, which is the finishedacrylate product stream, e.g., line 256. The distillate may be refluxedand the residue may be boiled up as shown.

Stream 254 comprises acetic acid and some acrylic acid. The fourthresidue exits fourth column 246 in line 256 and comprises a significantportion of acrylate product. As such, stream 256 is a finished productstream. Exemplary compositional ranges for the distillate and residue offourth column 246 are shown in Table 8. Components other than thoselisted in Table 8 may also be present in the residue and distillate.

TABLE 8 FOURTH COLUMN Conc. (wt. %) Conc. (wt. %) Conc. (wt. %)Distillate Acrylic Acid 0.1 to 40    1 to 30   5 to 30 Acetic Acid  60to 99    70 to 90   75 to 85 Water 0.1 to 25   0.1 to 10   1 to 5Alkylenating Agent less than 1 0.001 to 1 0.1 to 1 Propionic Acid <100.001 to 5 0.001 to 1  Residue Acrylic Acid at least 85    85 to 99.9  95 to 99.5 Acetic Acid less than 15   0.1 to 10 0.1 to 5 Water lessthan 1 less than 0.1 less than 0.01 Alkylenating Agent less than 1 0.001to 1 0.1 to 1 Propionic Acid 0.1 to 10  0.1 to 5 0.5 to 3

Returning to FIG. 3, at least a portion of stream 254 is directed tofifth column 248. Fifth column 248 separates the at least a portion ofstream 254 into a distillate in line 258 and a residue in line 260. Thedistillate may be refluxed and the residue may be boiled up as shown.The distillate comprises a major portion of acetic acid. In oneembodiment, at least a portion of line 260 is returned, either directlyor indirectly, to reactor 206. The fifth column residue exits fifthcolumn 248 in line 260 and comprises acetic acid and some acrylic acid.At least a portion of line 260 may be returned to fourth column 246 forfurther separation. In one embodiment, at least a portion of line 258 isreturned, either directly or indirectly, to reactor 206. In anotherembodiment, at least a portion of the acetic acid-containing stream ineither or both of lines 258 and 260 may be directed to an ethanolproduction system that utilizes the hydrogenation of acetic acid.Exemplary compositional ranges for the distillate and residue of fifthcolumn 248 are shown in Table 9. Components other than those listed inTable 9 may also be present in the residue and distillate.

TABLE 9 FIFTH COLUMN Conc. (wt. %) Conc. (wt. %) Conc. (wt. %)Distillate Acrylic Acid 0.01 to 10  0.05 to 5 0.1 to 1   Acetic Acid  50 to 99.9    70 to 99.5 80 to 99 Water  0.1 to 25   0.1 to 15  1 to10 Alkylenating Agent less than 10 0.001 to 5 0.01 to 5   Propionic Acid0.0001 to 10  0.001 to 5 0.001 to 0.05  Residue Acrylic Acid   5 to 50   15 to 40 20 to 35 Acetic Acid   50 to 95    60 to 80 65 to 75 Water0.01 to 10  0.01 to 5 0.1 to 1   Alkylenating Agent less than 1 0.001 to1 0.1 to 1   Propionic Acid <10 0.001 to 5 0.001 to 1   

In cases where the acrylate product split unit comprises at least onecolumn, the column(s) may be operated at suitable temperatures andpressures. In one embodiment, the temperature of the residue exiting thecolumn(s) ranges from 90° C. to 130° C., e.g., from 95° C. to 120° C. orfrom 100° C. to 115° C. The temperature of the distillate exiting thecolumn(s) preferably ranges from 60° C. to 90° C., e.g., from 65° C. to85° C. or from 70° C. to 80° C. The pressure at which the column(s) areoperated may range from 1 kPa to 300 kPa, e.g., from 10 kPa to 100 kPaor from 40 kPa to 80 kPa. In preferred embodiments, the pressure atwhich the column(s) are operated is kept at a low level e.g., less than50 kPa, less than 27 kPa, or less than 20 kPa. In terms of lower limits,the column(s) may be operated at a pressures of at least 1 kPa, e.g., atleast 3 kPa or at least 5 kPa. Without being bound by theory, it hassurprisingly and unexpectedly been found that by maintaining a lowpressure in the columns of acrylate product split unit 234 may inhibitand/or eliminate polymerization of the acrylate products, e.g., acrylicacid, which may contribute to fouling of the column(s).

It has also been found that, surprisingly and unexpectedly, maintainingthe temperature of acrylic acid-containing streams fed to acrylateproduct split unit 234 at temperatures below 140° C., e.g., below 130°C. or below 115° C., may inhibit and/or eliminate polymerization ofacrylate products. In one embodiment, to maintain the liquid temperatureat these temperatures, the pressure of the column(s) is maintained at orbelow the pressures mentioned above. In these cases, due to the lowerpressures, the number of theoretical column trays is kept at a lowlevel, e.g., less than 10, less than 8, less than 7, or less than 5. Assuch, it has surprisingly and unexpectedly been found that multiplecolumns having fewer trays inhibit and/or eliminate acrylate productpolymerization. In contrast, a column having a higher amount of trays,e.g., more than 10 trays or more than 15 trays, would suffer fromfouling due to the polymerization of the acrylate products. Thus, in apreferred embodiment, the acrylic acid split is performed in at leasttwo, e.g., at least three, columns, each of which have less than 10trays, e.g. less than 7 trays. These columns each may operate at thelower pressures discussed above.

The inventive process further comprises the step of separating analkylenating agent stream to form a purified alkylenating stream and apurified acetic acid stream. The purified alkylenating agent streamcomprises a significant portion of alkylenating agent, and the purifiedacetic acid stream comprises acetic acid and water. The separation ofthe alkylenating agent from the acetic acid may be referred to as the“acetic acid split.”

Returning to FIG. 3, alkylenating agent stream 234 exits alkylenatingagent split unit 204 and is directed to acetic acid split unit 242 forfurther separation, e.g., to further separate the alkylenating agent andthe acetic acid therefrom. Acetic acid split unit 242 may comprise anysuitable separation device or combination of separation devices. Forexample, acetic acid split unit 242 may comprise at least one column,e.g., a standard distillation column, an extractive distillation columnand/or an azeotropic distillation column. In other embodiments, aceticacid split unit 242 comprises a precipitation unit, e.g., a crystallizerand/or a chiller. Preferably, acetic acid split unit 242 comprises astandard distillation column as shown in FIG. 3. In another embodiment,acetic acid split unit 242 comprises a liquid-liquid extraction unit. Ofcourse, other suitable separation devices may be employed either aloneor in combination with the devices mentioned herein.

In FIG. 3, acetic acid split unit 242 comprises sixth column 250. Aceticacid split unit 242 receives at least a portion of alkylenating agentstream in line 234 and separates same into a sixth distillate comprisingalkylenating agent in line 262, e.g., a purified alkylenating stream,and a sixth residue comprising acetic acid in line 264, e.g., a purifiedacetic acid stream. The distillate may be refluxed and the residue maybe boiled up as shown. In one embodiment, at least a portion of line 262and/or line 264 are returned, either directly or indirectly, to reactor206. At least a portion of stream in line 264 may be further separated.In another embodiment, at least a portion of the acetic acid-containingstream in line 264 may be directed to an ethanol production system thatutilizes the hydrogenation of acetic acid form the ethanol.

The stream in line 262 comprises alkylenating agent and water. Thestream in line 264 comprises acetic acid and water. Exemplarycompositional ranges for the distillate and residue of sixth column 250are shown in Table 10. Components other than those listed in Table 10may also be present in the residue and distillate.

TABLE 10 SIXTH COLUMN Conc. (wt. %) Conc. (wt. %) Conc. (wt. %)Distillate Acrylic Acid less than 1 0.001 to 5    0.001 to 1    AceticAcid less than 1 0.001 to 5    0.001 to 1    Water 40 to 80 50 to 70 55to 65 Alkylenating Agent 20 to 60 30 to 50 35 to 45 Propionic Acid lessthan 1 0.001 to 5    0.001 to 1    Residue Acrylic Acid less than 1 0.01to 5   0.1 to 1   Acetic Acid 25 to 65 35 to 55 40 to 50 Water 35 to 7545 to 65 50 to 60 Alkylenating Agent less than 1 0.01 to 5   0.1 to 1  Propionic Acid less than 1 0.001 to 5    0.001 to 1   

In cases where the acetic acid split unit comprises at least one column,the column(s) may be operated at suitable temperatures and pressures. Inone embodiment, the temperature of the residue exiting the column(s)ranges from 90° C. to 130° C., e.g., from 95° C. to 120° C. or from 100°C. to 115° C. The temperature of the distillate exiting the column(s)preferably ranges from 60° C. to 90° C., e.g., from 65° C. to 85° C. orfrom 70° C. to 80° C. The pressure at which the column(s) are operatedmay range from 1 kPa to 500 kPa, e.g., from 25 kPa to 400 kPa or from100 kPa to 300 kPa.

The inventive process further comprises the step of separating thepurified acetic acid stream to form a second finished acetic acid streamand a water stream. The second finished acetic acid stream comprises amajor portion of acetic acid, and the water stream comprises mostlywater. The separation of the acetic from the water may be referred to asdehydration.

Returning to FIG. 3, sixth residue 264 exits acetic acid split unit 242and is directed to drying unit 238 for further separation, e.g., toremove water from the acetic acid. Drying unit 272 may comprise anysuitable separation device or combination of separation devices. Forexample, drying unit 272 may comprise at least one column, e.g., astandard distillation column, an extractive distillation column and/oran azeotropic distillation column. In other embodiments, drying unit 272comprises a dryer and/or a molecular sieve unit. In a preferredembodiment, drying unit 272 comprises a liquid-liquid extraction unit.In one embodiment, drying unit 272 comprises a standard distillationcolumn as shown in FIG. 3. Of course, other suitable separation devicesmay be employed either alone or in combination with the devicesmentioned herein.

In FIG. 3, drying unit 272 comprises seventh column 252. Drying unit 272receives at least a portion of second finished acetic acid stream inline 264 and separates same into a seventh distillate comprising a majorportion of water in line 266 and a sixth residue comprising acetic acidand small amounts of water in line 268. The distillate may be refluxedand the residue may be boiled up as shown. In one embodiment, at least aportion of line 268 is returned, either directly or indirectly, toreactor 206. In another embodiment, at least a portion of the aceticacid-containing stream in line 268 may be directed to an ethanolproduction system that utilizes the hydrogenation of acetic acid formthe ethanol.

Exemplary compositional ranges for the distillate and residue of seventhcolumn 252 are shown in Table 11. Components other than those listed inTable 11 may also be present in the residue and distillate.

TABLE 11 SEVENTH COLUMN Conc. (wt. %) Conc. (wt. %) Conc. (wt. %)Distillate Acrylic Acid less than 1 0.001 to 5  0.001 to 1  Acetic Acidless than 1 0.01 to 5 0.01 to 1 Water 90 to 99.9   95 to 99.9   95 to99.5 Alkylenating Agent less than 1 0.01 to 5 0.01 to 1 Propionic Acidless than 1 0.001 to 5  0.001 to 1  Residue Acrylic Acid less than 10.01 to 5 0.01 to 1 Acetic Acid 75 to 99.9   85 to 99.5   90 to 99.5Water 25 to 65     35 to 55   40 to 50 Alkylenating Agent less than 1less than 0.001 less than 0.0001 Propionic Acid less than 1 0.001 to 5 0.001 to 1 

In cases where the drying unit comprises at least one column, thecolumn(s) may be operated at suitable temperatures and pressures. In oneembodiment, the temperature of the residue exiting the column(s) rangesfrom 90° C. to 130° C., e.g., from 95° C. to 120° C. or from 100° C. to115° C. The temperature of the distillate exiting the column(s)preferably ranges from 60° C. to 90° C., e.g., from 65° C. to 85° C. orfrom 70° C. to 80° C. The pressure at which the column(s) are operatedmay range from 1 kPa to 500 kPa, e.g., from 25 kPa to 400 kPa or from100 kPa to 300 kPa. FIG. 2 also shows tank 276, which, collects at leastone of the process streams prior to recycling same to reactor 206. Tank276 is an optional feature. The various recycle streams that may,alternatively, be recycled directly to reactor 206 without beingcollected in tank 276.

Examples Example 1

A liquid-liquid extraction column using an extraction agent mixturecomprising diisobutylketone and cyclohexane was used to separate a crudeacrylate product. The diisobutylketone and cyclohexane were mixed at aweight ratio of approximately 9:1. This ratio is merely demonstrativeand it is expected that other ratios would yield similar results.

A crude acrylate product stream comprising approximately 26.4 wt %acrylic acid; 40.1 wt % acetic acid; 8.9 wt % formaldehyde; and 28.2 wt% water was fed to a liquid-liquid extraction column. The flow rates ofthe separation process streams are shown below.

Agitator Crude Feed, Solvent Feed, Raffinate, Extract, Run SPM g/ming/min g/min g/min 1 165 15.1 11.6 3.1 23.4 2 150 19.6 15.3 4.6 30.2

The following extractions were achieved. Extractions reflect thepercentage of component that is extracted from the crude acrylateproduct stream into the extract stream.

Run Acrylic Acid Acetic Acid Formaldehyde Water 1 98.6% 88.6% 43.7%47.1% 2 98.8% 87.9% 40.0% 44.4%

Comparative Example A

A liquid-liquid extraction similar to that of Example 1 was conductedwith the exception that only one extraction agent, diisobutylketone, wasemployed. A crude acrylate product streams comprising approximately 27.8wt % acrylic acid; 37.1 wt % acetic acid; 7.4 wt % formaldehyde; and27.6 wt % water was fed to a liquid-liquid extraction column. The flowrates of the separation process streams are shown below.

Agitator Crude Feed, Solvent Feed, Raffinate, Extract, Run SPM g/ming/min g/min g/min A-1 150 19.9 15.1 2.5 32.8 A-2 150 27.1 21.4 3.6 45.5A-3 175 27.7 21.4 4.3 44.9

The following extractions were achieved. Extractions reflect thepercentage of component that is extracted from the crude acrylateproduct stream into the extract stream.

Run Acrylic Acid Acetic Acid Formaldehyde Water A-1 99.5% 96.7% 63.7%65.5% A-2 99.5% 96.5% 60.5% 62.2% A-3 99.5% 95.6% 54.4% 60.2%

As shown by these examples, the use in an extraction of multipleextraction agents provides for improved separation of impurities from acrude acrylic product stream. For example, by operating the extractionin accordance with the present invention, extraction of formaldehydeinto the extract stream is reduced by approximately 30%, on average.Similarly, extraction of acetic acid into the extract stream is reducedby approximately 8%, on average. Also, extraction of water into theextract stream is reduced by approximately 27%, on average. Theseimprovements are achieved with minimal reductions (if any) in acrylicacid extraction.

While the invention has been described in detail, modifications withinthe spirit and scope of the invention will be readily apparent to thoseof skill in the art. In view of the foregoing discussion, relevantknowledge in the art and references discussed above in connection withthe Background and Detailed Description, the disclosures of which areall incorporated herein by reference. In addition, it should beunderstood that aspects of the invention and portions of variousembodiments and various features recited below and/or in the appendedclaims may be combined or interchanged either in whole or in part. Inthe foregoing descriptions of the various embodiments, those embodimentswhich refer to another embodiment may be appropriately combined withother embodiments as will be appreciated by one of skill in the art.Furthermore, those of ordinary skill in the art will appreciate that theforegoing description is by way of example only, and is not intended tolimit the invention.

We claim:
 1. A process for producing an acrylate product, the processcomprising the steps of: (a) providing a crude product stream comprisingthe acrylate product, an alkylenating agent, and water; and (b)contacting at least a portion of the crude product stream or aderivative thereof with an extraction agent mixture comprising at leasttwo extraction agents to form an extract stream comprising acrylateproduct and extraction agent and a raffinate stream comprisingalkylenating agent and water.
 2. The process of claim 1, wherein theextract stream comprises alkylenating agent in an amount less than 54%of the total alkylenating agent in the crude product stream.
 3. Theprocess of claim 1, wherein extract stream comprises water in an amountless than 60% of the total water in the crude product stream.
 4. Theprocess of claim 1, wherein extract stream comprises acrylate product inan amount greater than 75% of the total acrylate product in the crudeproduct stream.
 5. The process of claim 1, wherein the extract streamcomprises less than 30 wt % water.
 6. The process of claim 1, whereinthe extract stream comprises less than 20 wt % alkylenating agent. 7.The process of claim 1, wherein the raffinate stream comprises less than15 wt % acrylate product.
 8. The process of claim 1, wherein step (b) isperformed via liquid-liquid extraction.
 9. The process of claim 1,wherein step (b) is carried out at a temperature lower than 49° C. 10.The process of claim 1, wherein the at least two extraction agents areselected from the group consisting of diisobutyl ketone, cyclohexane,toluene, isopropyl acetate, o-xylene, p-xylene, m-xylene, butyl acetate,butanol, methyl acetate, methyl acrylate, diphenyl ether, ethylacrylate, isopropyl acetate, ethyl propionate, hexane, benzene,diisopropyl ether, n,n-dimethyl aniline, dibutyl ether, tetralin, butylacrylate, 2-ethylhexyl alcohol, isophorone, ditolyl ether, dimethylphthalate, 3,3 trimethyl-cyclohexanone, biphenyl, o-dichlorobenzene, andcombinations thereof.
 11. The process of claim 1, wherein the at leasttwo extraction agents are selected from the group consisting of ethersand alcohols.
 12. The process of claim 1, wherein the at least twoextraction agents comprise diisobutyl ketone and cyclohexane.
 13. Theprocess of claim 1, wherein the extraction selectivity to alkylenatingagent is less than 0.9%.
 14. The process of claim 1, wherein the massratio of acrylate product to alkylenating agent in the extract stream isgreater than 5:1.
 15. The process of claim 1, wherein the mass ratio ofacrylate product to alkylenating agent in the raffinate stream is lessthan 0.02:1.
 16. The process of claim 1, further comprising the step of:separating the raffinate strain to form an alkylenating agent stream anda first solvent stream.
 17. The process of claim 16, wherein step (b) isconducted in an extraction unit and wherein the first solvent stream isrecycled to the extraction unit.
 18. The process of claim 1, furthercomprising the step of: separating the extract stream to form anintermediate product stream and a second solvent stream.
 19. The processof claim 18, wherein step (b) is conducted in an extraction unit andwherein the second solvent stream is recycled to the extraction unit.